1. Field of the Invention
The present invention relates to methods of forming semiconductor devices. More particularly, the present invention relates to methods of cleaning substrates after forming tungsten patterns.
2. Description of Related Art
As semiconductor devices become more highly integrated, design rules become gradually smaller. As the design rule is reduced, an interconnection of the semiconductor device decreases in width, causing an increase in resistance of the interconnection. Unfortunately, when the design rule is reduced, an interconnection such as a gate line or a bit line, which is composed of polysilicon or polycide, cannot possibly meet characteristics required for semiconductor devices.
To solve the problem of high resistance of interconnections, metals of low resistance are frequently used as interconnections for highly integrated semiconductor devices. Aluminum has been conventionally used for interconnections, while tungsten, a refractory metal, has been widely used for such interconnections as bit lines. Advantageously, when the bit line is composed of tungsten, a bit line contact plug is also composed of tungsten, which exhibits a reliable gap fill characteristic and does not increase an interface resistance between the bit line and the contact plug.
To form a tungsten plug in a semiconductor device, a contact hole is formed in an interlayer dielectric layer (ILD) to expose a portion of a lower conductive region. Tungsten is sufficiently stacked using CVD on the semiconductor substrate where the contact hole is formed, such that the contact hole is filled with tungsten. Then, a contact plug is formed by performing a tungsten chemical mechanical polishing (W-CMP) or an etchback process on the resultant substrate until the ILD is exposed. Also, when forming a tungsten interconnection pattern using a damascene process, the W-CMP is performed until the ILD is exposed.
In the event that the etchback process is carried out to form a tungsten pattern, metal polymers including tungsten are generated in large quantities and become attached to the substrate and the like. The metal polymers should be removed through a subsequent cleaning process. Similarly, when the W-CMP is carried out to form the tungsten pattern, oxidized slurry and tungsten residue usually remain on the substrate. The slurry residue should necessarily be removed because it may cause defects to the substrate.
In conventional methods, a cleaning process is carried out using a mixed solution of dilute ammonium hydroxide and dilute fluoric acid to remove the metal polymers and slurry residue after forming the tungsten pattern. A scrubbing process, which is a mechanical process, is performed on one or both sides of the substrate. However, the conventional cleaning process and scrubbing process are inefficient, and oxidized slurry residue and metal polymers cannot be removed to required thicknesses. As a result, the oxidized slurry residue and metal polymers remain on the substrate, thus inducing defects such as a single bit fail due to a short 21 between a capacitor contact plug 13 and a bit line 11, a bridge between bit lines, and the like, as shown in an electron micrograph of FIG. 1.